This invention relates to a method of manufacturing doped silica glass and a method of manufacturing an optical fiber preform by utilizing the resulting doped silica glass manufactured by the former method.
A doped silica glass containing principally GeO.sub.2 as a dopant and optionally diphosphorous pentoxide (P.sub.2 O.sub.5), diboron trioxide (B.sub.2 O.sub.3) and the like as additional dopants has been employed as a material for optical fiber.
Heretofore, this type of the production of doped silica glass has been carried out mainly in accordance with the following three types of soot processes:
(1) CVD Process (Inside Vapor Phase Oxidation Method) (see U.S. Pat. No. 4,217,027)
In this process, the side of a quartz tube is heated by a flame at a temperature of about 1,500.degree.-1,700.degree. C., and SiCl.sub.4 and a compound of dopant, e.g., GeCl.sub.4 supplied into the quartz tube in gaseous phase is subjected to thermal oxidation to form a doped silica glass layer. In this case, SiCl.sub.4 and GeCl.sub.4 become SiO.sub.2 and GeO.sub.2 glass fine particles and at the same time, form a transparent GeO.sub.2 doped silica glass body by the thermal oxidation reaction. Such operation is repeated to obtain a desired thickness of the doped silica glass layer. Then, when the flame is permitted to attain a temperature of about 1,700.degree.-1,800.degree. C., the quartz tube shrinks to make the glass layer solid, thereby producing an optical fiber preform. In this process, the synthesis of silica glass fine particles, addition of GeO.sub.2 and vitrification of the silica glass fine particles are simultaneously carried out in accordance with thermal oxidation reaction by the same heat source.
(2) OVPO Method (Outside Vapor Phase Oxidation Method) (see U.S. Pat. No. 3,859,073)
In this method, fine glass particles consisting of SiO.sub.2 and GeO.sub.2 synthesized in a flame are jetted on the side of a rotating starting material (mandrel) to obtain a porous silica glass sintered body containing GeO.sub.2. The resulting hollow round bar-like porous silica glass sintered body is heated and vitrified by means of a ring-like heater element at a temperature of about 1,500.degree.-1,600.degree. C., thereby to obtain a transparent GeO.sub.2 doped silica glass body. This GeO.sub.2 doped silica glass body is inserted into a quartz tube to obtain an optical fiber preform. In the method, the synthesis of silica glass fine particles, addition of GeO.sub.2 and sintering are simultaneously effected by the same heat source, whilst only vitrification is carried out in accordance with a separate step.
(3) VAD Method (Vapor-Phase Axial Deposition Method) (see U.S. Pat. No. 4,062,665)
Glass forming raw materials such as SiCl.sub.4, GeCl.sub.4 and the like as well as an O.sub.2 -H.sub.2 flame stream consisting of H.sub.2, O.sub.2 and insert gases are blown off from a synthesizing torch connected to a feed pipe for the glass forming raw materials and a feed pipe for H.sub.2 -O.sub.2 gases, whereby the aforesaid glass forming raw materials are subjected to flame hydrolysis to produce glass fine particles like SiO.sub.2, GeO.sub.2 and at the same time, the fine glass particles are sintered by the same flame stream to form a porous glass body. This porous glass body is successively deposited on the extreme end of a supporting rod being moved upwardly by pulling up the same while rotating by means of a rotary pulling-up device to fabricate a porous silica glass sintered body containing GeO.sub.2. Then, the resulting porous silica glass sintered body is heated and fused by means of a heater element disposed on the upper portion of the apparatus at a temperature of about 1,500.degree.-1,600.degree. C. to effect vitrification, and as a result, a transparent GeO.sub.2 doped silica glass body is produced. The resultant GeO.sub.2 doped silica glass body is inserted into a quartz tube to use the same as an optical fiber preform. In this method, the synthesis of fine glass particles, addition of GeO.sub.2 and sintering are simultaneously carried out by the same heat source similarly to that in the above outside vapor phase oxidation method, and on the other hand, the deformation and virtification steps are carried out by means of the ring-like heater element disposed on the upper portion of the apparatus.
Simple explanations have been made about three typical methods for manufacturing doped silica glass for optical fiber fabrication which are practised at present, but these conventional methods have the following various disadvantages.
First, in a conventional method (soot process) for manufacturing doped silica glass, increasing the amount of glass forming raw materials supplied per unit time and therefore, the rate of production of the doped silica glass, decreases efficiency of synthesizing the glass fine particles by flame hydrolysis. Besides, since the synthesis of glass fine particles, addition of GeO.sub.2 and sintering are simultaneously carried out by the same heat source, when the amount of glass forming raw materials is increased, the sintering becomes insufficient so that formation of the porous glass body becomes difficult.
According to the study by the present inventors, it was found that because of the limitation as mentioned above, it was difficult to obtain 500 g or more of production per unit time in accordance with the manufacturing method of the doped silica glass by employing the soot process and furthermore, efficiency of 80% in the production thereof was the upper limit in such process.
In order to avoid the disadvantages of the soot process as to increasing the rate of fabrication of a glass body, arising when a transparent glass body is directly produced from fine glass particles (a so-called direct vitrification process), GeO.sub.2 cannot be added to the transparent glass body, and as a result, doped silica glass cannot be obtained.
In case of these conventional methods, synthesis of fine glass particles, addition of GeO.sub.2 and sintering have simultaneously been effected by the same heat source, and in case of inside vapor phase oxidation method, even vitrification has simultaneously been made with the above other steps by the same heat source. For this reason, adjusting the conditions suitable for synthesis of fine glass particles, addition of GeO.sub.2 and sintering thereof, has been difficult. Thus, increase in the rate of production of doped silica glass which is, homogeneous and transparent doped cannot be obtained.
For instance, in order to improve the rate of production in an inside vapor phase oxidation method, when the amount of SiCl.sub.4 and GeCl.sub.4 was increased (SiCl.sub.4 /GeCl.sub.4 ratio being constant), there arose such a problem that the vitrification was not sufficient, and porous glass sintered bodies remained as a lamellar constituents. Then, when the flame temperature was further raised to accelerate the reaction and at the same time to form perfectly a transparent glass body, there occurred a problem that a ratio of GeO.sub.2 content in the formed doped silica glass layer decreased. In order to improve the rate of production and to obtain doped silica glass having desired characteristics in CVD method, a fine adjustment of the conditions for synthesizing glass fine particles is required, for adding GeO.sub.2 and for effecting vitrification so as to set optimum synthesizing conditions. Therefore, there has naturally been a limitation for improving the rate of production. A similar tendency can also be observed in the OVPO method and the VAD method, respectively. That is, there was a disadvantage in that when the amounts of SiCl.sub.4 and GeCl.sub.4 were increased, the degree of sintering in the formed porous sintered body was decreased to generate "cracking" etc., so that a porous glass sintered body for an optical fiber preform could not be produced.
On the other hand, in order to improve the above stated disadvantage, when the flame is intensified, there arose a problem that the GeO.sub.2 content was decreased. Thus, simple adjustment of fabrication conditions, in order to increase the rate of production of doped silica glass is not possible.
The above discussion may also be applied for the case where a doped silica glass is produced by utilizing PbO.sub.2 or SnO.sub.2 as a dopant.
According to these conventional methods, however, when it is intended that the amount of glass-forming raw materials etc. per unit time is increased and the rate of production of doped silica glass is increased, efficiency for synthesizing glass fine particles decreases and at the same time, the sintering thereof becomes insufficient, so that it is difficult to form a porous glass body. On the other hand, for the sake of improving the efficiency for synthesizing glass fine particles and making the sintering sufficient, when it is contemplated that the temperature of the oxy-hydrogen flame is raised and the rate of the production of doped silica glass is improved, no dopant PbO.sub.2 or SnO.sub.2 can be added.
For these reasons as mentioned above, these conventional methods could not avoid such disadvantages that doped silica glass must be produced matching the rate in production of the doped silica glass and the amount of a dopant which is added, so that the amount of PbO.sub.2 or SnO.sub.2 which can be added to the glass fine particles was a very minor amount, while the rate of the production was also slow.